Re: [Joey Coyle <joey@xcoyle.com>] restriction

ht@cogsci.ed.ac.uk (Henry S. Thompson) writes:

> From: Joey Coyle <joey@xcoyle.com>
> Subject: restriction
> To: ht@cogsci.ed.ac.uk
> Cc: dbeech@us.oracle.com
> Date: Thu, 25 May 2000 15:31:09 -0400
> 
> There are many in the ASN.1 world that want to move to XML Schema, and if I am
> reading correctly, it seems you have said no to many.
> 
> 
> When modeling by restriction, which is quite common, you have jumbled the
> meaning of minOccurs.  It means both that instances must contain data for that
> element, and secondly that derived types by restriction must include that
> element.
> 
> 
> This is not logical.  There are many circumstances when you want an elemental
> data slot filled in a parent type instance, and then don't even want the
> element in a derived child.

I'm not aware of any OO or related type system in which derived types
(== subclasses) may eliminate methods/properties of base
types/superclasses.  The XML Schema type definition derivation design
is based on the requirement that processes which consume members
of the base type will work with members of the derived types, which
means required properties cannot be lost.

> 
> ##################################################
> 
> The next issue I would like to address is about the level of
> restriction.  Can you explain this:
> 
> 
> A complex type with an empty specification for {final} can be used as a {base
> type definition} for other types derived by either of extension or
> restriction; the explicit values extension, and restriction prevent further
> derivations by extension and restriction respectively. If all values are
> specified, then the complex type is said to be [Definition:]  final: no
> further derivations are possible.
> 
> 
> Does it mean that a type which is derived by restriction and is instantiable,
> can not be used as a base type for further restriction?

I'm sorry I don't see the connection between the above two
paragraphs.  By 'instantiable' do you mean not "abstract='true'"?
A type definition validly derived by restriction can be used as the
base for further derivation by restriction unless it is
"final='restriction ...'".

ht

-- 
  Henry S. Thompson, HCRC Language Technology Group, University of Edinburgh
          W3C Fellow 1999--2001, part-time member of W3C Team
     2 Buccleuch Place, Edinburgh EH8 9LW, SCOTLAND -- (44) 131 650-4440
	    Fax: (44) 131 650-4587, e-mail: ht@cogsci.ed.ac.uk
		     URL: http://www.ltg.ed.ac.uk/~ht/

Received on Friday, 26 May 2000 05:14:48 UTC